1. Field of Invention
This invention relates generally to the field of semiconductor materials and processing, and more particularly, to thin film epitaxy of compound semiconductors at low temperatures, and most particularly, to III–V compound semiconductors such as gallium arsenide.
2. Description of the Prior Art
Numerous organic and inorganic materials behave as electrical semiconductors. Among the inorganic semiconductors, elemental semiconductors such as silicon have formed the basis for much of modern integrated circuit technology. However, compound semiconductors comprising more than a single chemical species can have interesting optical and/or electronic properties not obtainable in conventional semiconducting materials such as silicon. In particular, gallium arsenide (GaAs) is an example of a direct bandgap semiconductor in which, roughly speaking, electronic transitions can occur between the top of the valence electronic energy band and the bottom of the conduction electronic energy band without the necessity for a concurrent change in the momentum of the transitioning electron(s).
Electronic transitions in direct bandgap materials are typically very efficient and, among other things, typically give such materials interesting and useful optical properties. As a result, GaAs is an important material in many optoelectronic applications such as light-emitting-diodes, semiconductor lasers, among others.
In addition, the electronic structure and properties of GaAs offer advantages in terms of the fabrication of high-speed electronic devices. Therefore, GaAs also is finding application in the highest-speed electronic devices for use in conjunction with high-speed computing, high-speed data processing, high frequency microwaves, among other applications.
GaAs exists in the form of bulk crystals, but can also be deposited as high quality thin films by molecular beam epitaxy (MBE), organmetallic vapor phase epitaxy (OVPE), among other deposition techniques. Such deposition techniques permit the deposition and formation of GaAs thin films but also, and significantly, permit the deposition of alloys of GaAs. That is, a thin film of GaAs is deposited along with one or more additional components to form a thin film compound or alloy. Typical GaAs alloys comprise ternary compounds in which a third component is included, such as aluminum (forming AlGaAs), indium (forming InGaAs) as well as dilute nitrides GaAsN containing small amounts of nitrogen. Ga is typically substituted by aluminum (Al) and/or indium (In) while As is typically substituted by nitrogen (N), phosphorous (P) and/or antimony (Sb) resulting in a group of III–V compounds or alloys, including ternary alloys such as GaAsP, GaAsSb, among others. While quaternary alloys (such as InGaAsN, InGaAsP) and even quinternary alloys (GaInAsNPb) are known, our primary concern herein will be directed towards the ternary alloys of III–V compounds. Generally, such ternary alloys derived from GaAs are only obtained in thin film depositions, adding to the practical interest of thin film GaAs in the fabrication of practical electronic and optoelectronic devices.
Typically, thin film GaAs is deposited (or grown) at temperatures of approximately 500 deg. C. to approximately 600 deg. C., although many GaAs films are grown at a temperature close to approximately 580 deg. C., the temperature that is typically sufficient to evaporate the top oxide cover layer. Ternary alloys are typically grown at different temperatures: AlGaAs typically at a higher temperature than GaAs while InGaAs typically at lower temperature. Under such conditions, impurities from the ambient conditions in the deposition chamber and/or residing in the source material(s) typically are included in the GaAs film, tending to degrade its properties. For example, inclusion of such impurities in the GaAs film typically leads to films having a carrier concentration in the range of approximately 1014 to 1015 carriers/(cubic centimeter-cc) or more. That is, typical GaAs thin films are conductive, either n-type conductive or p-type conductive depending on the particular impurity. Generally such films are n-type conductive since impurities tend to be electron-donors, but p-type conductive films are also known. Conductive GaAs thin films are disfavored in the fabrication of electronic devices, tending to allow unwanted and deleterious electronic interaction between components and devices intended to be electrically isolated.
It has been observed that GaAs deposited at low temperature (LT-GaAs) can lead to the formation of thin films having much improved resistivity. That is, LT-GaAs deposited typically in the temperature range from approximately 200 deg. C. to approximately 400 deg. C. can have carrier concentrations as low as approximately 108 to 109 carriers/cc, a factor of about a million improvement (increase) in resistivity (or decrease in conductivity). Thus, LT-GaAs would be favored over conventional GaAs films as a base on which to fabricate electronic devices, leading to much improved device isolation.
Note, however, that for films grown near or below approximately 300 deg. C., conductivity may be undesirably high despite low carrier concentration due to “hopping conduction” arising from direct electron hopping from defect to defect when a high concentration of defects is present. Hopping conduction can be cured to produce a high resistivity film by a suitable annealing step, typically annealing the film at temperatures around or exceeding approximately 400 deg. C.
In addition to higher resistivity, it has been observed that LT-GaAs has a much shorter carrier lifetime following optical excitation. That is, electromagnetic radiation impinging on GaAs typically causes electronic transitions and the creation of mobile carriers of electronic charge. LT-GaAs has the advantageous property that such carriers disappear (are trapped) within an extremely short time, perhaps as short as picoseconds (10−12 sec) or less. Such ultrashort carrier lifetimes are advantageously used in very high-speed electronic devices, perhaps operating in the range of terahertz (1012 Hz).
However, fully utilizing the favorable properties of LT-GaAs in practical devices faces several challenges. The formation of LT-GaAs is typically not highly reproducible. Seemingly minor changes in processing conditions can lead to markedly different properties in the ultimate film of LT-GaAs.
Additionally, use in practical electronic devices requires that LT-GaAs survive subsequent processing with its properties intact or, at a minimum, with its properties altered in predictable and reproducible ways. In particular, processing steps used in typical electronic device fabrication often occur at temperatures greatly exceeding the deposition temperatures of LT-GaAs. Such high temperatures often change the physical, chemical and electrical properties of LT-GaAs so as to render it unsuitable for its intended use.
In view of the foregoing, a need exists in the art for methods for the production of LT-GaAs in a controlled and reproducible manner and for producing forms of LT-GaAs capable of surviving high-temperature or other processing with favorable properties. The present invention relates to methods for doping-assisted control of defects in LT-GaAs to achieve these and other objectives, and to the forms of LT-GaAs thereby produced.